Your past actions are the best predictor of your future decisions. That'swhy data about your buying decisions and demographic is so valuable: it enablescompanies to know what and when you'll buy. Marketing has focused on influencingyour future decisions because changing your past actions, or the memory of them,has seemed impossible. You can't change the past. That's no longer true.Marketers now have the technology to meddle with your memories. To find out how,let's first step back to old-school product placement.

Dr Pepper comesto mind when I hear the word " Spider-Man". In the film, as Peter Parker learnsto use his powers, he spies a can of Dr Pepper across the room and, after a fewtries, gets to enjoy drinking the cold beverage that had been out of reach.Mass-media product placement of this kind works by association and memory. Butthis is a last-century technique. The goods and services famous people consumewill continue to wriggle into our subconscious, but increasingly we live in aworld where there isn't one source of truth. The internet has diversified ourcommunication channels. Narrowcasting is replacing broadcasting; our friends arereplacing celebrities.

In this new world of narrowcasting, productplacement is about to get uncomfortably personal. This is how it is going tohappen: each of our worlds is the sum of our experiences. Our memories helpdefine who we are. We document and share that world through our status updatesand photo albums. What we share creates the documentary of our lives. The onlinepresence of your friends and family -- their e-autobiography -- is becoming yoursource of truth about their lives. The value of the trust we place in ourfriends and the ease with which our autobiographies can be modified will not gounexploited.

The goal is digital revisionist history -- productsinjected into our memories. Tweaking photos on Flickr and Facebook to change thedrink we are holding to a can of Budweiser, the billboard in the background toSamsung's, your friend's T-shirt to Abercrombie. It might be Facebook's nextbillion-dollar business model. And it might not always be so inane as infectingyour buying habits -- it may include your political views.

Cognitivepsychology has shown that our memories are predictably fallible -- reconstructedfrom bare-bone frameworks, not remembered in high resolution. PsychologistElizabeth Loftus has proved that eyewitness reports are easily manipulated viathe "misinformation effect": leading questions or misleading information canchange critical details of memories. In studies, Loftus and others found thatafter a single session they could implant a detailed false memory into one infour people. In a recent study at University of California Irvine, published inApplied Cognitive Psychology, researchers were able to change participants'understanding of famous events by showing them doctored photos. Memory is easilyreshaped.

As you look back at your photos from a year ago, you get thesubliminal message: "I did have a great time at that party and, yes, I guess Iwas drinking Budweiser." Your memory has been hijacked. If you don't consciouslyremember or don't know which photos have been tampered with, the heist isflawless. Even if you do remember, your friends probably won't know anydifferent. Budweiser has just exploited their trust in you. Next time they goshopping, that little memory does its brand-affinity magic and changes theirbehaviour.

But why would you let anyone manipulate your pictures andhijack your memory? Perhaps a company might offer to touch up your pictures --remove spots, make you look thinner -- in exchange for product placement. Youget benefit, companies get value and you'll rarely know which photos have hadwhat added to them. Your memory, your past, is now monetisable. You can generatevalue just from having lived.

Our memories are still pristine, but notfor long. We know that the best predictor of our future decisions is our pastactions. With digital revisionist history, those past actions aren't immutableand marketers will be writing our personal histories. The question is: how canwe stop it?